Rotating recorder with dual encoder arrangement having eccentricity compensation

ABSTRACT

A rotating recording device, such as an electron beam recorder, is provided with a dual encoder arrangement. A first encoder is employed as a spindle motor controller and located at a first end of a spindle. A second encoder is mounted at a turntable adjacent to a recording surface and used as a position, velocity or clock source for recording the pattern on the substrate. Eccentricity of the mounting of the second encoder is measured against the more accurately mounted spindle control encoder and compensated by a digital clock generating system using a digital phase locked loop.

FIELD OF THE INVENTION

The present invention relates to the field of recording information witha rotatable recording device, and more particularly, to the control of arotatable recording device employing a dual encoder arrangement.

BACKGROUND OF THE INVENTION

A magnetic disk drive, such as a hard disk drive, stores data on one ormore disks coated with a magnetic medium. For read/write purposes, thesurface of the magnetic medium carries a number of generally paralleldata tracks, which on a disk type medium, are arranged concentricallywith one another about the center of the disk.

An actuator arm positions a transducer or “head” over a desired track,and the head writes data to the track or reads data from the track. Asthe disk rotates, the actuator arm moves the head in a radial directionacross the data tracks under control of a closed-loop servo system,based on position information or “servo data”, which is stored withindedicated servo fields of the magnetic medium of the disk. The servofields can be interleaved with data sectors on the disk surface or canbe located on a separate disk surface that is dedicated to storing servoinformation. As the head passes over the servo fields, it generates afeedback signal that identifies the location of the head relative to thecenterline of the desired track. Based on this location, the servosystem moves the actuator arm to adjust the head's position so that itmoves toward a position over the desired track and/or a desired locationwithin the track of current interest.

One requirement in the manufacture of such a hard disk drive relates tothe formation of the servo patterns on the magnetic disk, which aretypically in concentric circular patterns. Mastering Systems for formingthe servo tracks on a master stamper used in magnetic contact printinghave used both stepped translation mechanisms with laser beams andcontinuous translation mechanisms with electron beams. During therecording of the servo data on the substrate, the substrate is locatedon a rotating turntable located at the top of a spindle. A spindlecontrol motor rotates the spindle in accordance with control signalsprovided by a controller. The servo tracks are recorded through exposureto an electron beam or laser beam.

A format signal generator is used to control the electron beam generatoror laser beam generator to form the pattern on the disk as it isrotating with the turntable. The control of the format signal generator,and hence the recording on the disk, may be made in accordance withencoder signals from an encoder located at the motor.

Typically, the encoder is provided at a bottom end of a spindle, the endopposite to that of the spindle on which the turntable is mounted. Aprecise motor control is provided by employing an encoder located inthis position. In other words, the encoder at the bottom of the spindlemay be mounted such that there is substantially no eccentricity withrespect to the axis of rotation of the spindle. This allows for a veryprecise control of the motor based on the encoder signals.

Although providing for a precisely centered mounting and accurate motorcontrol signals, the location of the encoder at the bottom of the motoris problematic when used to provide a clock, position or velocity sourcefor the format signal generation process during recording. This is dueto the mechanical vibrations, however slight, that occur in the rotatingportions of the recording system. In particular, the vibrations at thetop of the spindle, where the turntable is located, are not synchronizedwith the vibrations at the bottom of the spindle, where the encoder islocated. Because the distances employed in servo tracks are extremelysmall, such as between 50 to 90 nm, even minute disturbances will createproblems of track-to-track phase errors.

Simply moving the encoder adjacent to the recording surface at the topof the spindle does not provide an adequate solution, however. This isbecause, in practice, providing for a substantially perfectlycentrically mounted encoder adjacent to the recording surface has provento be very difficult to achieve. Hence, an encoder provided adjacent tothe recording surface at the turntable exhibits eccentricity duringrotation so that the signals are inadequate for providing a preciseclock, position or velocity signal employed to control recording.

SUMMARY OF THE INVENTION

There is a need for an arrangement that provides for a precise controlof the spindle motor used in rotating the turntable that supports arecording media, but further provides for a precise clock, position orvelocity signal that overcomes the disadvantages of single encodersystems.

This and other needs are met by embodiments of the present inventionwhich provide an electron beam recorder comprising a spindle motor and aspindle driven by the spindle motor. The spindle extends to the spindlemotor and has first and second ends. A turntable is mounted at thesecond end of the spindle and has a central axis of rotation. A firstencoder is mounted at the first end of the spindle, and a second encoderis mounted on the turntable. A controller is coupled to the first andsecond encoders and a spindle motor, and controls the spindle motor andgenerates recording clock signals as a function of encoder signalsreceived from the first and second encoders.

By using dual encoders, the spindle motor may be precisely controlled asa function of encoder signals from the first encoder, and recordingclock signals are provided as a function of the encoder signals receivedfrom the second encoder. In certain embodiments, the controller includeslogic for compensating for the eccentric mounting of the second encoderon the turntable with respect to the central axis of rotation of theturntable.

The earlier stated needs are also met by other embodiments of thepresent invention which provide a method of controlling a rotatingrecording device having a spindle, a first encoder mounted on a firstend of the spindle, a turntable mounted on a second end of the spindle,a second encoder mounted on the turntable, and a controller coupled tothe first and second encoders and controlling the rotating recordingdevice in accordance with signals from the first and second encoders.The method comprises the steps of determining an eccentricity of thesecond encoder with respect to a central axis of rotation of thespindle, and compensating for the determined eccentricity in the signalsfrom the second encoder.

The earlier stated needs are also met by other embodiments of thepresent invention which provide an electron beam recorder comprising aturntable and motor arrangement with an encoder mounted at theturntable, and means for compensating signals from the encoder caused byeccentricity in the mounting of the encoder.

The forgoing and other features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an electron beam type recorder system, forforming servo marks on a disk workpiece, with certain elements of theelectron beam device shown in cross-section.

FIG. 2 is a cross-section of a portion of the electron beam recordingdevice of FIG. 1, but showing in more detail the motor, controller andencoder configuration in accordance with embodiments of the presentinvention.

FIG. 3 is a block diagram of eccentricity compensation logic constructedaccording to an embodiment of the present invention, for use in theelectron beam recorder of FIGS. 1 and 2.

FIG. 4 shows a flow chart of a method of controlling a rotatingrecording device, such as the electron beam recorder of FIGS. 1-2 inaccordance with embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention addresses and solves problems related to theprecise recording of information, such as servo information, on arecording disk. Problems created by the vibrations in such recordingsystems are overcome in embodiments of the invention by the provision ofa dual encoder system. One of the encoders may be employed to controlthe spindle motor, while the other encoder may be used to provide clocksource signals (or position or velocity) signals used by the signalformat generator in the recording device. Eccentricity compensationlogic allows the precise clock signal to be generated from the encodersignals provided by an encoder that is mounted on a turntable, despiteeccentric mounting of the encoder in this location. Hence, a precisecontrol of the spindle motor and precise generation of a clock signalfor recording is achieved, despite presence of vibrations in the system.

FIG. 1 depicts a cross-sectional side view of a portion of an electronbeam recorder or other rotating recording device in accordance withembodiments of the present invention. Details regarding the dual encoderarrangement are depicted in FIG. 2 and will be discussed later. Marksare formed on the disk surface 21 by the beam recording system 10. Therecording system 10 produces a relative motion between the diskworkpiece 21 and a recording beam. During this movement, the beamimpacts a surface and a system modulates the beam to expose the surfaceof the disk in a desired pattern. The beam, for example, is modulated toform servo marks in a photoresist surface on the disk 21, according toservo mark patterns.

The system 10 is an electron beam recording system, but may also beanother type of recording system, such as a laser beam recording system,for example, which would include different components. The system 10includes a turntable 31 and an electron beam column 33 for generatingand manipulating the recording beam. The turntable 31 supports the disk21 for rotation in the direction B about its vertical axis. Examples ofsuch an electron beam recording system are available from Unaxis NimbusLimited. It is assumed that those skilled in the art are generallyfamiliar with the structure and operation of electron beam columndevices. However, for completeness of the discussion, a summarydescription thereof is provided.

In the example, the electron beam column 33 includes a thermal fieldemission (TFE) electron source 35 and a suppression assembly 37. Thecolumn may also include electronic extractor 39. When appropriatevoltages are applied to the TFE source 35, the suppression assembly 37and the extractor 39, these elements cooperate to generate a stream ofelectrons for further processing in the column 33. The stream ofelectrons passes through a first triple element lens 41, then throughblanking plates 43 and a blanking aperture 45. The stream of electronsthen passes through one or more additional lenses, represented, forexample, by the second triple lens 47 in the drawing.

The beam position may be controlled by application of a voltage to thedeflection plates 49. The precise location of the deflection plates inthe column, relative to the other elements of the column, is notcritical. In the example, the deflection plates are between the blankingaperture 45 and the lens 41, although other column structures use otherarrangements.

The shapes of and voltages of the signals applied to the elements ofcolumn 33 serve to focus and shape the stream of electrons into amodulated beam of a desired shape and having a desired energy level fora particular application. For example, a set of signals applied to theelements of the column 33 causes the column to generate a modulated beamfor forming servo patterns of particular size and depth at locations onthe surface of the disk 21. The drawing in FIG. 1 shows the beamtraveling through the column 33 as a straight line, for convenience ofillustration. In actual operation, the beam would converge and divergeas it passes through the various elements of the column 33, in order tofocus on a sample on the turning table 31 in a desired manner.

The electron beam recording system 30 also includes a format signalgenerator 61, for generating the various signals used by the electronbeam column 33 to modulate and deflect the beam and thus format thepatterns being exposed on the disk 21. The formatter 61 essentiallycomprises circuitry forming one or more signal generators, for producingthe various signals applied to the components of the column 30 toproduce the desired beam.

One example of a signal produced by the generator 61 is the formatmodulation signal (or beam “format” signal) for application to theblanking plates 43, which controls the energy level of the electron beamand thus the exposure of the recorded pattern. A controller 30 controlsthe rotational speed of the turntable 31. The format signal generator 61provides an encoder signal to the turntable control 63, to regulate therotational operations of the turntable 31, and the control 63 mayprovide one or more feedback signals to the generator 61 indicatingturntable position and/or speeds. For example, the turntable control 63may provide an index signal each time a mark or a feature on theturntable or disk passes a reference point. The index signal providesinformation regarding speed of rotation. For example, the number ofindex pulses per minute indicates the number of revolutions per minute(RPM). The time between pulses of the index signal represents a periodof one rotation. The angle between rotation start point (e.g., 12o'clock) and the reference point is a known constant. Hence, the indexcan also be used to determine start and end points of successiverotations.

As discussed earlier, the use of a single encoder centrically mounted ata bottom end of a spindle motor does not provide accurate enoughinformation due to vibrations at the top of the spindle, where theturntable is mounted. This leads to track-to-track phase errors. The useof a dual encoder arrangement, as depicted in the embodiment of FIG. 2,overcomes these disadvantages.

In FIG. 2, the elements of column 33 are depicted as a single block forease of illustration. The spindle motor 14 has coils 16 and a spindle 18that rotates on a central axis. The spindle 18 is supported by spindlebearings 20.

A first encoder 22 is positioned at the first end of the spindle 18(i.e., the bottom end of the spindle 18). A second encoder 24 is locatedadjacent to the recording surface 31 of the turntable, i.e., at thesecond end of the spindle 18. Sensors 26 operate in conjunction with thefirst and second encoders 22, 24 to provide encoder signals to thecontroller 30.

The controller 30 includes speed control logic 32, digital clock logic34 and eccentricity compensation logic 36. The speed control logic 32controls the speed of the spindle motor 14. As will become apparent, thespeed control logic 32, during recording, normally controls the speed ofthe spindle motor 14 based on signals received from the sensor 26 at thefirst encoder 22.

The digital clock logic provides its output signals to the format signalgenerator to control the electronic beam recorder column 33 to producean electron beam in accordance with clock signals. As will be explainedfurther, the digital clock logic 34 employs the eccentricitycompensation logic 36 to compensate for the eccentricity of the mountingof the second encoder 24 at the turntable 31. The use of signals derivedfrom the second encoder 24 through the sensor 26 allow for a moreprecise clock signal to be generated due to the proximity of the secondencoder 24 to the recording surface 21, rather than relying on signalsrelated to the first encoder 22. The eccentricity compensation logic 36employs stored information, such as eccentricity information stored intable 38, to perform the eccentricity compensation.

In order to use the second encoder 24 as a position, velocity or clocksource for recording the pattern, such as the servo pattern, on the disksubstrate 21, the eccentricity of this second encoder 24 must bemeasured. In certain embodiments of the present invention, theeccentricity of mounting of the second encoder 24 is measured againstthe more accurately mounted first encoder 22. A process for measuringand compensating for the eccentricity is depicted in FIG. 4 according tocertain embodiments of the present invention. In step 70, the process isstarted. In step 72, control of the spindle motor 18 is made accordingto the signals related to the first encoder 22. Measurements are made ofthe repeatable component of the timing of the pulses, using the firstencoder 22. All encoders have a tolerance, i.e., how tightly they cancontrol distances between marks laid down on a glass substrate. Once themarks are laid down on the disk, these marks are fixed, but a distancevariation, (frequency component when the encoder is rotated), reveals avariation in the timing of the pulses, which is fixed relative to anindex position. It is termed a “repeatable component” because the amountof timing variation referenced in the index will always be the same.

In step 74, the second encoder 24 is employed to control the speed ofthe spindle motor 14 and is also used to measure the repeatablecomponent analogously to the control and measurements performed by thefirst encoder 22 in step 72.

Next, in step 76, the first encoder 22 is employed to control thespindle motor 14, but the second encoder 24 is used to measure therepeatable component of the timing of the pulses.

In step 78, based upon the above measurements, a measure of eccentricityof the mounting of the second encoder 24 is determined. A table may thenbe formulated with this eccentricity information, in step 80. The tablethus contains stored information related to the eccentricity of themounting of the second encoder 24, with respect to the central rotationaxis of the turntable 31.

In step 82, recording is performed with the relatively accuratelymounted first encoder 22 operating as the spindle control encoder forthe spindle motor 14, and the digital clock generating system providingsignals to the format signal generator 61 during recording. Since thesecond encoder 24 is located adjacent to the recording surface 21 in theturntable 31, it can provide a more accurate clock signal than the firstencoder 22 located a much greater distance away from the recordingsurface 21 and turntable 31, thereby avoiding the significant mechanicalerrors and momentary displacements that can occur between the substrateand the encoder. This prevents the pattern being recorded on thesubstrate from being inaccurate. At the same time, however, theeccentricity of the mounting of the second encoder 24 is compensatedusing the eccentricity compensation logic 36 and the stored informationin the table 38.

The digital clock logic 34 and the eccentricity compensation logic 36 isdepicted in block diagram form in FIG. 3. Logic 34 and 36 receive theencoder signals from the first and second encoders 22, 24. During anactual recording operation, the signals from the second encoder 24 areemployed to produce the digital clock source, as described above.

The digital clock logic 34 and eccentricity compensation logic 36include a digital phase detector 40 coupled to a custom filter (loopcompensator) 42. A numerically controlled oscillator 44 receives theoutput of the filter 42 and provides its output to the input of adivide-by-N multiplier 46 that is used to multiply the clock. Thedigital clock logic 34 and eccentricity compensation logic 36 are ableto reference the table 38 to provide the eccentricity compensation asrequired.

With the embodiments of the present invention, accurate recording of ahigh quality pattern on a master substrate or other media is achievablein an electron beam recorder or other rotating recording device, by theuse of a dual encoder arrangement. The encoder of the spindle controlsystem located some distance away from the recording surface is notemployed as the primary clock source for recording the pattern on themaster substrate, since the mechanical distance between the spindleencoder and the substrate surface will allow significant mechanicalerrors and momentary displacements to occur between the substrate andthe encoder, causing this pattern recorded on the substrate to beinaccurate. Instead, the invention provides a second encoder located onthe turntable adjacent to the recording surface, and which can be usedas a position, velocity or clock source for recording the pattern on thesubstrate. A mounting eccentricity of the second encoder can becompensated by measuring the eccentricity against the spindle controlencoder, with a digital clock generating system using a digital phaselocked loop compensating for this measured eccentricity.

Although the present invention has been described and illustrated indetail, it is to be clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the scope of the present invention being limited only by theterms of the appended claims.

1. An electron beam recorder, comprising: a spindle motor; a spindledriven by the spindle motor, the spindle extending through the spindlemotor through the spindle motor and having first and second ends; aturntable mounted at the second end of the spindle and having a centralaxis of rotation; a first encoder mounted at the first end of thespindle; a second encoder mounted on the turntable; and a controllercoupled to the first and second encoders and the spindle motor andcontrolling the spindle motor and generating recording clock signals asa function of encoder signals received from the first and secondencoders.
 2. The electron beam recorder of claim 1, wherein thecontroller includes logic for compensating for eccentric mounting of thesecond encoder on the turntable with respect to the central axis ofrotation of the turntable.
 3. The electron beam recorder of claim 2,wherein the controller includes speed control logic for controllingspeed of the spindle motor.
 4. The electron beam recorder of claim 3,wherein the controller includes digital clock generating logic forgenerating a digital clock signal.
 5. The electron beam recorder ofclaim 4, further comprising an electron beam generator and a formatsignal generator coupled to the electron beam generator and modulatingthe output of the electron beam generator.
 6. The electron beam recorderof claim 5, wherein the format signal generator is coupled to receivethe digital clock signal as a input and modulates the output of theelectron beam generator as a function of the digital clock signal. 7.The electron beam recorder of claim 6, wherein the speed control logicis controlled as a function of encoder signals from only the firstencoder.
 8. The electron beam recorder of claim 7, wherein the digitalclock generating logic generates the digital clock signal as a functionof encoder signals from only the second encoder.
 9. The electron beamrecorder of claim 8, wherein the logic for compensating for eccentricmounting includes stored information regarding a measured eccentricityof the mounting of the second encoder.
 10. The electron beam recorder ofclaim 9, wherein the digital clock generating logic includes a phaselocked loop (PLL) coupled to the logic for compensating for eccentricmounting of the second encoder according to the stored information. 11.The electron beam recorder of claim 5, wherein the controller includesposition signal generating logic that generates a position signal as afunction of encoder signals from only the second encoder, wherein theformat signal generator is coupled to receive the position signal as aninput and modulates the output of the electron beam generator as afunction of the position signal.
 12. The electron beam recorder of claim5, wherein the controller includes velocity signal generating logic thatgenerates a velocity signal as a function of encoder signals from onlythe second encoder, wherein the format signal generator is coupled toreceive the velocity signal as an input and modulates the output of theelectron beam generator as a function of the velocity signal.
 13. Amethod of controlling a rotating recording device having a spindle, afirst encoder mounted on a first end of the spindle, a turntable mountedon a second end of the spindle, a second encoder mounted on theturntable, and a controller coupled to the first and second encoders andcontrolling the rotating recording device in accordance with signalsfrom the first and second encoders, the method comprising: determiningan eccentricity of the second encoder with respect to a central axis ofrotation of the spindle; and compensating for the determinedeccentricity in the signals from the second encoder.
 14. The method ofclaim 13, wherein the step of determining includes: controlling rotationof the spindle in accordance with signals from the first encoder, andmeasuring a repeatable component of a timing signal with the firstencoder; controlling rotation of the spindle in accordance with signalsfrom the second encoder, and measuring a repeatable component of atiming signal with the second encoder; and controlling rotation of thespindle in accordance with signals from the first encoder, and measuringa repeatable component of a timing signal with the second encoder. 15.The method of claim 14, wherein the step of determining further includesforming a table based on the measurings, the table containing storedinformation relating to the determined eccentricity.
 16. The method ofclaim 15, wherein the step of compensating includes passing signals fromthe second encoder through a phase locked loop and correcting thesignals from the second encoder using the stored information in thetable.
 17. The method of claim 16, wherein during a recording operationthe signals from the first encoder control rotation of the spindle andthe corrected signals from the second encoder are used as sourcesignals.
 18. The method of claim 17, wherein the source signals are atleast one of: a position signal source; a velocity signal source; and aclock signal source.
 19. The method of claim 18, wherein the rotatingrecording device is an electron beam recorder.